MOSCOW (AFP) - A Russian opposition lawmaker who attended the secret funeral of a soldier apparently killed in Ukraine was in hospital on Saturday after what he said was a politically motivated attack.

Speaking from his hospital bed, Lev Shlosberg, who had been investigating Russian soldiers' presence in Ukraine, linked the assault late on Friday to his probe.

He told AFP some 100 paratroopers based in his northwestern town of Pskov had been killed in combat in Ukraine.

"An entire company was killed," the 51-year-old leader of a regional branch of the opposition party Yabloko told AFP by telephone, citing figures given to him by the soldiers' families.

Russia denies claims it has deployed regular troops to Ukraine.

Earlier this week, Shlosberg, who is also a journalist for a local newspaper, attended the burial of a soldier near Pskov.

On Friday evening he was assaulted by three unidentified assailants in an attack that left him hospitalised with head and eye injuries and a concussion, his aide Alexander Zakharov said.

Shlosberg said he and colleagues from Yabloko knew that at least three soldiers apparently killed in Ukraine had been buried near Pskov this month.

The lawmaker suggested that the slain soldiers were being buried across Russia in secret. After a handful of media reported on the Pskov funerals, the name tags were removed from the men's graves, said independent TV channel Dozhd.

Russian military commanders had imposed a virtual blackout on any information about the deployment of servicemen, Shlosberg and Zakharov said.

Relatives of the soldiers had been threatened not to speak to media, saying otherwise the men may not come back alive, Shlosberg said.

Outside a military base in the central Russian town of Kostroma on Thursday, AFP witnessed a similar situation, with army wives being discouraged from speaking to media.

Valeria Sokolova, the only soldier's wife who agreed to be interviewed by AFP in Kostroma, said some 350 soldiers from the town had been sent this month to the border with Ukraine and had gone incommunicado.

Commanders have refused to specify their whereabouts, only saying the soldiers are "not in Russia", said Sokolova, adding that several body bags had come back this week.

She has not answered her phone since Friday.

The commander of Russia's paratroopers, Vladimir Shamanov, has told reporters that "everyone is alive and well".

Shlosberg does not remember the details of the assault.

"The memory of that has completely disappeared," he told AFP. "They attacked me from behind, the people who did this are professionals."

His party also linked the attack to his investigation.

"I believe that attack on Lev Shlosberg is connected to his investigation of the deployment of Pskov paratroopers to Ukraine," Yabloko leader Grigory Yavlinsky said on Twitter.

Shlosberg pointed the finger at Putin. "It's his war," he said.

A criminal probe has been opened, prosecutors said. The Organisation for Security and Cooperation in Europe also called for a full investigation, noting that there have been several recent attacks on journalists in the region.

Kiev and the West say regular Russian troops are on the ground in Ukraine fighting alongside separatists, who this week staged a lightning counter-offensive that has turned the tide in the nearly five-month conflict.

Russia is conducting military drills near the border with Ukraine, but officials have repeatedly denied that its troops have been deployed to its former Soviet neighbour.

Rights groups and opposition leaders called on Putin to stop what they dubbed an "undeclared war".

"We demand that Russian aggression against Ukraine be immediately halted," the country's oldest rights group, Memorial, said in a statement.

Writing in local newspaper Pskovskaya Gubernia this week, Shlosberg said Russia was in the grips of a "genuine fratricidal war".

"How many people with Ukrainian roots are among Russian servicemen? How many people with Russian roots are among Ukrainian servicemen?"